Replace #ifdef HAVE_UNISTD_H checks with #ifdef G_OS_UNIX

In Windows development environments that have it, <unistd.h> is mostly
just a wrapper around several other native headers (in particular,
<io.h>, which contains read(), close(), etc, and <process.h>, which
contains getpid()). But given that some Windows dev environments don't
have <unistd.h>, everything that uses those functions on Windows
already needed to include the correct Windows header as well, and so
there is never any point to including <unistd.h> on Windows.

Also, remove some <unistd.h> includes (and a few others) that were
unnecessary even on unix.

https://bugzilla.gnome.org/show_bug.cgi?id=710519

gio/gdbusmessage.c

@@ -29,9 +29,6 @@
 #include <errno.h>
 #include <sys/types.h>
 #include <sys/stat.h>
-#ifdef HAVE_UNISTD_H
-#include <unistd.h>
-#endif
 
 #if MAJOR_IN_MKDEV
 #include <sys/mkdev.h>